This invention relates generally to motor controls and more specifically to a start-up control for a switched reluctance motor.
Although they have been known for some time, interest in switched reluctance motor (SRM) drives has recently revived. Compared to conventional induction and synchronous motor drive systems, the SRM drive is simple in construction and economical. In addition, the converter which supplies power to the SRM machine requires fewer power devices and, therefore, is more economical and reliable. In view of these advantages, the switched reluctance motor drive system provides an attractive alternative to conventional drive systems and is expected to find wide popularity in industrial applications.
Switched reluctance motors conventionally have multiple poles or teeth on both the stator and rotor (i.e. doubly salient). There are phase windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator poles is connected in series to form one phase of the multiphase switched reluctance motor.
Torque is produced by switching current on in each phase winding in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be applied by a converter using unidirectional current switching elements such as thyristors or transistors.
In operation, each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the stator winding of that phase, providing energy from a DC supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate toward a minimum reluctance configuration and partly into stored energy associated with the magnetic field. After the switch is opened, part of the stored magnetic energy is converted to mechanical output and part of the energy is returned to the DC source.
The control requirements of the SRM drive are so unique that concepts of induction and synchronous type machines can hardly be extrapolated to the SRM. Previously, SRM drives, in running condition, were controlled open loop with angle and current amplitude regulation by manual adjustment, and the controls usually employed discrete components and dedicated hardware. Such prior control systems were often bulky, complex, expensive, limited in mode of operation, and hardware intensive. Although suitable for laboratory tests, they did not readily lend themselves to industrial application.
Recently, a programmable, closed-loop, four-quadrant control system incorporating feedback control, angle control and current control has been developed. This new control system is described in a paper entitled "Microcomputer Control of Switched Reluctance Motor" by B. K. Bose et al., published in the Conference Record of the October 1985 IEEE Industrial Application Society Annual Meeting and is the subject of commonly assigned, co-pending U.S. patent application Ser. No. 915,288 filed concurrently herewith. The paper, which appears at pages 542-547 of the 1985 Annual Meeting Conference Record is incorporated herein by reference, for background purposes.
The present invention relates to a start-up control which can advantageously be employed with the overall control system described in the above-mentioned paper. A switched reluctance motor, unlike an induction motor, is not easy to start. If current pulses are applied in sequence to the stator windings at any arbitrary position of the rotor, the SRM may randomly start either in forward or reverse direction. In the past, switched reluctance motors have been started with complex, discrete digital logic circuits from signals provided by a rotor position encoder. The starting technique was unreliable and could not set automatically the direction of motion. Further, the prior start-up systems were unable to smoothly transition to a feedback mode because in the past the drive control was designed to operate in open-loop condition.
A need thus exists for a start-up control for a switched reluctance motor which overcomes the drawbacks of present-day designs, is compatible with a feedback control system, and facilitates use of the switched reluctance motor for general purpose industrial applications.